a. Field of the Invention
This invention relates to liquid recording medium suitable for use in a recording method which performs image recording by ejecting the liquid recording medium from an orifice of a nozzle in the form of droplets and sputtering the same onto an image recording member. More particularly, the invention is concerned with such liquid recording medium having remarkably improved heat response, thermal efficiency, stability, continuous recording performance over a long period of time, and various other characteristics.
b. Description of Prior Arts
So-called non-impact recording methods have recently drawn public attention, because unconfortable noises during the recording operation could be reduced to a negligible order. Among these particularly important is the so-called ink jet recording method which allows high-speed recording on a plain paper without particular image-fixing treatment, and, in this particular field, there have been proposed various approaches including those already commercialized, and others still under development.
Such ink jet recording method, in which droplets of a liquid recording medium (usually called "ink") are impinged and deposited on an image recording member to achieve the recording, can be classified into several processes according to the method of generating the droplets, and also to the method of controlling the direction of impingement of the droplets.
The first process is disclosed, for example, in the U.S. Pat. No. 3,060,429 (Teletype process), in which the liquid droplets are generated by electrostatic pull, and the droplets thus generated on demand are deposited on an image recording member with or without an electric-field control on the sputtering direction.
More specifically, the electric-field control is achieved by applying an electric field between the liquid contained in a nozzle having an orifice and an accelerating electrode, thereby causing the liquid to be emitted from the orifice and to sputter between x-y deflecting electrodes which are so arranged as to controlling the electric field according to recording signals, and thus selectively controlling the sputtering direction of the droplets according to variations in strength of the electric field to attain ink deposition at desired positions.
The second process disclosed, for example, in the U.S. Pat. No. 3,596,275 (Sweet process) and U.S. Pat. No. 3,298,030 (Lewis and Brown process), in which flow of liquid droplets of controlled electrostatic charges is produced by continuous vibration and is sputtered between deflecting electrodes forming a uniform electric field therebetween to attain image recording on a recording member.
More specifically, in this second process, a charging electrode which receives recording signals is provided in front of, and at a certain distance from, an orifice of a nozzle constituting a part of a recording head equipped with a piezo vibrating element, and a pressurized liquid is supplied into the nozzle, while an electric signal of a determined frequency is applied to the piezo vibrating element to cause mechanical vibration thereof, thereby causing the orifice to emit a flow of liquid droplets. As the emitted liquid is charged by electrostatic induction by the above-mentioned charging electrode, each droplet has a charge corresponding to the recording signal. The droplets having such controlled charge are subjected to deflection corresponding to the amount of the charge during this sputtering in a uniform electric field between the deflecting electrodes in such a manner that only those droplets carrying the recording signals are deposited onto the recording member.
The third process is disclosed, for example, in the U.S. Pat. No. 3,416,153 (Hertz process), in which an electric field is applied between a nozzle and an annular charging electrode to generate a mist of liquid droplets by continuous vibration. In this process, strength of the electric field applied between the nozzle and charging electrode is modulated according to the recording signals to control atomization of liquid for gradation in the recorded image.
The fourth process disclosed, for example, in the U.S. Pat. No. 3,747,120 (Stemme process) is based on a principle fundamentally different from that used in the foregoing three processes.
In contrast to the abovementioned three processes, in which the image recording is achieved by electrically controlling the liquid droplets emitted from the nozzle during their sputtering, thus selectively depositing only those carrying the recording signals onto the recording member, the Stemme process is featured in generating and impinging the droplets only when they are required for recording.
More specifically, in this Stemme process, electric recording signals are applied to a piezo vibrating element provided in a recording head having a liquid-emitting orifice, and the recording signals are converted into mechanical vibration of the piezo element, by which vibrating the liquid droplets are emitted from the orifice and deposited onto a recording member.
The foregoing four processes, while they have their respective advantages, are accompanied by drawbacks which are inevitable or have to be prevented.
The foregoing first to third processes rely on electric energy for forming droplets or droplet flow of liquid recording medium, and also on an electric field for controlling deflection of the droplets. For this reason, the first process, though structurally simple, requires a high voltage for droplet formation and is not suitable for high-speed recording, since a multi-orificed recording head is difficult to make.
The second process, though being suitable for high-speed recording as the use of multi-orifice structure in the recording head is feasible, inevitably results in a structural complexity and is further accompanied by other drawbacks such as necessity for precise and difficult electric controls for governing the impinging direction of droplets, and tendency to result in formation of satellite dots on the recording member.
The third process, though advantageous in achieving recording of an improved gradation by atomization of the emitted droplets, has drawbacks of difficulty in controlling the state of atomization, presence of background fog in the recorded image and being unsuitable for high-speed recording because of difficulty in constructing a multi-orificed recording head.
In comparison with the foregoing three processes, the fourth process has relatively important advantages such as a simpler structure, absence of liquid recovery system, since the droplets are emitted on demand from the orifice of the nozzle in contrast to the foregoing three processes wherein the droplets having no contribution to the image recording have to be recovered, and a larger freedom in selecting the materials constituting the liquid recording medium not requiring electric conductivity in contrast to the first and second processes wherein the medium has to be conductive. On the other hand, the fourth process is again accompanied by drawbacks such as difficulty in obtaining a small head or a multi-orificed head, because the mechanical working of head is difficult and also because a small piezo vibrating element of a desired frequency is extremely difficult to obtain, and inadequacy for high-speed recording, because the emission and impingement of the liquid droplets have to be effected by the mechanical vibrating energy of the piezo element.
As explained in the foregoing, each of the conventional processes has its own advantages and drawbacks in connection with the structure, applicability to high-speed recording, fabrication of recording head, in particularly, multi-orificed one, formation of satellite dots, and formation of background fog. Therefore, their use has been limited to the fields, in which such advantages can only be exploited.
In addition to several characteristics suitable for manuscript and printing such as ordinary offset printing, the liquid recording medium which is generally called "ink-composition" and is utilized in the abovementioned kinds of recording method has been given other characteristics in accordance with the modes of recording as described above.
For instance, these kinds of recording method should be excellent in signal response and faithfulness in reproduction, since the recording operation is done by ejecting droplets of the liquid recording medium from an orifice of a nozzle. Further, these recording methods are required to have various other characteristics such that the liquid recording medium can flow through the nozzle at a speed in conformity to the recording speed, that fixation of the liquid recording medium onto an image recording member such as paper, etc. is quick upon recording, that satisfactory image density can be obtained in the recorded image, and that the liquid recording medium has long storage life, and various others. Furthermore, when electric or electrostatic method is adopted for the liquid ejecting method, or for controlling the sputtering direction of the droplets of the liquid recording medium, such electric or electrostatic characteristics are also taken into consideration to the abovementioned properties. In order therefore to satisfy such various characteristics, those properties such as viscosity, surface tension, resistivity, electrical capacitance, dielectric constant, etc. of the liquid recording medium are properly regulated to desired values.
Thus, while the conventional liquid recording medium has been given various characteristics, they have been limited to the category of hydrodynamic and electrical characteristics in view of the nature of the recording modes, or, in view of the fact that the direct energy to eject and sputter the liquid recording medium from the orifice of the nozzle in the form of droplets is electrical or mechanical.
As the totally improved recording method which has removed various disadvantages in the conventional recording methods as mentioned in the foregoing, there has already been proposed an improved recording method in U.S. Patent Application Ser. No. 948,236 of the same assignee as designated in the present application.
Therefore, the abovementioned conventional liquid recording medium is not necessarily satisfactory as the liquid recording medium for such improved recording method according to the present invention, in which the direct energy to cause the liquid recording medium to be ejected and sputtered from the orifice of the nozzle in the form of droplets is thermal energy. For example, in case the recording operation is conducted at a particularly high speed with the conventional liquid recording medium, the heat response (i.e., responsive property to the action of heat energy) of the liquid recording medium is not sufficient with the result that there may take place such occasion that stability in the formation of the droplet becomes deficient. Further, in a device having extremely fine nozzle, orifice, etc., there arise not infrequently such inconveniences that, when the continuous recording is conducted over a long period of time, feeding rate of the liquid recording medium in the nozzle is reduced to become unable to cope with the recording speed, or the heat response of the liquid recording medium lowers, or stability in the droplet formation reduces, or clogging occurs at the heat energy applying section of the nozzle, or others, whereby stabilized recording operation becomes difficult.
Thus, when the liquid recording medium which has so far been used is employed in the recording method as disclosed in the abovementioned U.S. Patent Application Ser. No. 948,236, there often take place various phenomena unfavorable to the stable recording operation with the consequence that high quality recorded image having high clarity, sufficient image density, and being free from satellite dots cannot be obtained, and the advantage in the abovementioned improved recording method cannot be fully utilized. In consequence of this, the characteristics to be imparted to the liquid recording medium which may be suitably adopted for the abovementioned recording method should be considered from an entirely different standpoint from the conventional liquid recording medium.
The present invention has been accomplished as the results of studies and researches on the aforementioned points of view.